1. Field of the Invention
The present invention relates to a liquid crystal display unit, and more particularly to an in-plane-switching (IPS) active-matrix liquid crystal display unit.
2. Description of the Related Art
Liquid crystal display (LCD) units are generally characterized by low-profile shapes, lightweight structures, and low-power requirements. Particularly, active-matrix liquid crystal display (AM-LCD) units which comprise a two-dimensional matrix of pixels energizable by active devices are highly promising as high-image-quality flat panel displays. Among those active-matrix liquid crystal display units which are finding widespread use are thin-film-transistor liquid crystal display (TFT-LCD) units which employ thin-film transistors (TFTs) used as active devices for switching individual pixels.
Conventional AM-LCD units utilize a twisted-nematic (TN) electrooptical effect, and comprise a liquid crystal layer sandwiched between two substrates. The liquid crystal layer is activated when an electric field is applied substantially perpendicularly to the substrates.
U.S. Pat. No. 3,807,831 discloses an in-plane-switching liquid crystal display unit having a liquid crystal layer which is activated when an electric field is applied substantially parallel to two substrates which sandwich the liquid crystal layer therebetween, the liquid crystal display unit including interleaved arrays of alternate parallel electrodes.
Japanese patent publication No. 21907/88 reveals an AM-LCD unit based on a TN electrooptical effect and including interleaved or interdigitating arrays of alternate parallel electrodes for the purpose of reducing parasitic capacitance between a common electrode and a drain bus line or between a common electrode and a gate bus line.
FIG. 1 of the accompanying drawings shows a conventional in-plane-switching liquid crystal display unit. The illustrated conventional liquid crystal display unit comprises a liquid crystal layer sandwiched between two glass substrates 11, 12, and interdigitating arrays of alternate parallel electrodes 70 mounted on one of the glass substrates 11. When a voltage is applied between the electrodes 70, a liquid crystal activating electric field E1 is generated parallel to the glass substrates 11, 12 and perpendicularly to the interdigitating teeth of the electrodes 70 for thereby changing the orientation of liquid crystal molecules 21. Therefore, the application of the voltage between the electrodes 70 is effective to control the transmittance of light through the liquid crystal layer. The term “the orientation of liquid crystal molecules” used in this specification means the direction of the longer axis of liquid crystal molecules.
With the in-plane-switching liquid crystal display unit shown in FIG. 1, it is necessary that when the voltage is applied, the liquid crystal molecules be rotated in a certain direction in order to achieve stable displays. To meet such a requirement, it is customary to initially orient the liquid crystal molecules in a direction that is slightly shifted from a direction perpendicular to the liquid crystal activating electric field. Specifically, the liquid crystal molecules are initially oriented at an angle of φLCO (<90°) with respect to a direction perpendicular to the parallel pairs of the interdigitating teeth of the electrodes. In the specification, the direction of the electric field and the orientation of the liquid crystal molecules will be described in the range of from −90° to 90° (the counterclockwise direction being positive) with respect to a reference direction (φ=0) which is perpendicular to the parallel pairs of the interdigitating teeth of the electrodes. As described later on, in order to accomplish sufficient display contrast, it is necessary to rotate the liquid crystal molecules 45° from the initial orientation. Therefore, it is preferable to orient the liquid-crystal molecules at an angle of φLCO in the range of 45°≦φLCO<90°. In the in-plane-switching liquid crystal display unit shown in FIG. 1, the initial orientation of the liquid crystal molecules is slightly shifted clockwise (as viewed from the upper substrate 12) from the parallel pairs of the interdigitating teeth of the electrodes. When the voltage is applied, therefore, the liquid crystal molecules are rotated clockwise as indicated by the arrows.
The transmittance T of light passing through the liquid crystal cell shown in FIG. 1 which is sandwiched between two confronting polarizers whose axes of polarization transmission (directions of polarization) are perpendicular to each other is expressed by the following equation (1):                     T        =                              1            2                    ⁢                      sin            2                    ⁢                      {                          2              ⁢                              (                                                      ϕ                    P                                    -                                      ϕ                    LC                                                  )                                      }                    ⁢                                    sin              2                        ⁡                          (                                                πΔ                  ⁢                                                                          ⁢                  nd                                λ                            )                                                          (        1        )            where φLC represents the orientation of the liquid crystal molecules when a voltage is applied thereto, φP the direction of the axis of transmission of the polarizer on which the light falls, Δn the refractive index anisotropy of the liquid crystal layer, d the thickness of the cell (the thickness of the liquid crystal layer, and λ the wavelength of the light. The direction φA of the axis of transmission of the polarizer from which the light exits is expressed by φA=φP+90° or φA=φP−90°. It is possible to control the transmittance of the light by varying the orientation φLC of the liquid crystal molecules with a liquid crystal activating electric field parallel to the substrates based on the above equation (1). If the direction of the axis of transmission of one of the polarizers and the initial orientation of the liquid crystal molecules are in agreement with each other (φLCO=φP or φLCO=φA), then the liquid crystal display unit is brought into a dark display state when no voltage is applied. If the orientation of the liquid crystal molecules is rotated substantially 45° under a liquid crystal activating electric field, then the transmittance becomes highest, and the liquid crystal display unit is brought into a bright display state. Of course, the polarizers may be so arranged that the liquid crystal display unit will be brought into a dark display state when a voltage is applied.
It has been assumed for the sake of brevity that the liquid crystal molecules in the liquid crystal layer between the upper and lower substrates are uniformly rotated. Discussions based on such a simplified model do not essentially affect the principles of the present invention. Actually, however, those liquid crystal molecules which are held in contact with the surfaces of the upper and lower substrates are relatively firmly fixed in position, and do not basically change their orientation, whereas those liquid crystal molecules which are positioned nearly intermediate between the upper and lower substrates change their orientation to a greater extent. In view of these practical considerations, the in-plane angle φLC through which the liquid crystal molecules rotate under an applied electric field is represented as a function of coordinates in the transverse direction of the liquid crystal layer.
In order to accomplish sufficient display contrast, the orientation of the liquid crystal molecules may be rotated substantially 45° in the entire liquid crystal layer. However, for the reasons described above, the liquid crystal molecules which are positioned nearly intermediate between the upper and lower substrates are actually rotated more than 45°.
Published Japanese translation No. 505247/93 of a PCT international publication (International publication No. WO91/10936) describes improvements of angle of view characteristics, which have been poor in TN liquid crystal display devices, achieved by the in-plane-switching liquid crystal display unit. Because of their excellent angle of view characteristics, in-plane-switching active-matrix liquid crystal display units have recently been considered as a candidate for large-size display monitors.
FIG. 2 of the accompanying drawings shows the transmittance of the liquid crystal display unit shown in FIG. 1 as it varies when the applied voltage is changed, with respect to various observational directions in which the liquid crystal display unit is observed. The observational directions are defined as φobs and θobs where φobs is an angle of orientation with respect to a direction perpendicular to the direction of the electrodes and θobs is an angle of tilt from a direction perpendicular to the substrates. A sample liquid crystal cell used in obtaining the measurements shown in FIG. 2 was arranged such that φLC=85°, φP=85°, and φA=−5°. The sample liquid crystal cell had interdigitating arrays of alternate parallel electrodes, including interdigitating teeth each having a width of 5 μm with adjacent ones of the interdigitating teeth being spaced 15 μm from each other. The sample liquid crystal cell had a liquid crystal material whose refractive index anisotropy Δn is 0.067. The sample liquid crystal cell had a thickness of 4.9 μm. It can be seen from FIG. 2 that the transmittance does not change largely depending on the observational direction. Therefore, the in-plane-switching liquid crystal display unit shown in FIG. 1 has excellent angle of view characteristics.
However, the in-plane-switching liquid crystal display unit shown in FIG. 1 suffers a problem in that displayed images may look bluish or reddish to a viewer depending on the observational direction.
FIG. 3 of the accompanying drawings shows the transmittance of the liquid crystal display unit shown in FIG. 1 as it varies with the wavelength with respect to various observational directions when the liquid crystal display unit is brought into a bright display state. The measurements shown in FIG. 3 were obtained from the same liquid crystal cell as the one used to obtain the measurements shown in FIG. 2. In the liquid crystal cell, the orientation φLC of the liquid crystal molecules is 40° because when the liquid crystal cell is brought into a bright display state, i.e., when a voltage is applied, the orientation φLC changes about 45° from the initial orientation φLCO=85°. It can be understood from FIG. 3 that when the liquid crystal cell is brought into a bright display state, the peak of the transmission spectrum at the observational direction φobs=40° is shifted toward shorter wavelengths, making displayed images bluish, and the peak of the transmission spectrum at the observational direction φobs=−50° is shifted toward longer wavelengths, making displayed images reddish. The same tendency was observed at those observational directions which are 180° spaced from the above observational directions.
As described above, while the in-plane-switching liquid crystal display unit has much better characteristics than the conventional TN liquid crystal display units with regard to display contrast and freedom from gradation reversal, it suffers the problem of tilts depending on the observational direction.
In the above liquid crystal cell, the liquid crystal molecules are directed at the initial orientation φLCO=85° in the absence of any applied voltage. When a voltage is applied to bring the liquid crystal cell into a bright display state, the orientation φLC of the liquid crystal molecules is 40° because the orientation φLC changes about 45° from the initial orientation φLCO=85°. The direction in which displayed images look bluish to the viewer corresponds to this orientation φLC of the liquid crystal molecules, and the direction in which displayed images look reddish to the viewer corresponds to the orientation perpendicular to the orientation φLC. In a display mode based on birefringence, as achieved by the above liquid crystal cell, light having a wavelength which satisfies the relationship of Δn·d=λ/2 passes most efficiently through the liquid crystal cell, as can be seen from the equation (1). The tinting depending on the angle of view, i.e., the angle at which the liquid crystal cell is observed, is caused by the dependency of the birefringence (Δn·d) of the liquid crystal layer on the angle of view.
The dependency of the birefringence of the liquid crystal layer on the angle of view will be described in detail below.
It is assumed that the angle formed between the direction of travel of light and the longitudinal direction of liquid crystal molecules is represented by θ2, the refractive index with respect to an ordinary ray of light which is vibrated (polarized) in a direction perpendicular to a direction called the optic axis of crystal is represented by no, and the refractive index with respect to an extraordinary ray of light which is vibrated (polarized) parallel to the optic axis is represented by ne. Effective refractive index anisotropy Δn′ when light is obliquely applied to the liquid crystal cell is given by the following equation (2):                               Δ          ⁢                                          ⁢                      n            ′                          =                                                            n                e                            ⁢                              n                o                                                                                                          n                    e                                    ⁢                                      cos                    2                                    ⁢                                      θ                    2                                                  +                                                      n                    o                                    ⁢                                      sin                    2                                    ⁢                                      θ                    2                                                                                -                      n            o                                              (        2        )            
When light is applied perpendicularly to the liquid crystal cell, since θ2=90°, the effective refractive index anisotropy Δn′ is given as Δn′=ne−no. In the direction in which displayed images look bluish to the viewer, because the angle of view is tilted to the longitudinal direction of liquid crystal molecules, the angle θ2 becomes θ2<90° and Δn′ becomes smaller. In the direction in which displayed images look reddish to the viewer, because the angle of view is tilted to a direction perpendicular to the longitudinal direction of liquid crystal molecules, the angle θ2 remains θ2=90° and Δn′=Δn. FIGS. 4A and 4B illustrate the refractive index anisotropy as it varies with the angle of view.
When light is applied obliquely to the liquid crystal cell, since the substantial thickness d′ of the liquid crystal layer is given by d′=d/cos θobs, the substantial thickness d′ becomes larger independent of the direction in which the angle of view is tilted.
Because of changes of both the refractive index anisotropy and the thickness of the liquid crystal layer, the birefringence (Δn′·d′) varies, changing the tint depending on the angle of view.
Table 1 shown below contains details of the tinting.
TABLE 1ΔndΔn · dRemarksBluish tintReducedIncreasedReduced*Reddish tintUnchangedIncreasedIncreased***The longitudinal direction of the liquid crystal molecules when the liquid crystal cell is in a bright display state.**The direction perpendicular to the longitudinal direction of the liquid crystal molecules.
As described above, the conventional in-plane-switching liquid crystal display units cannot avoid tinting of displayed images in certain directions.
In view of the experimental data and considerations described above, the inventors have made the present invention in efforts to suppress tinting in in-plane-switching active-matrix liquid crystal display units.